Nutrition and digestion · System
Bacteria in the gut
You are carrying around thirty trillion bacteria in your large intestine. Your immune system knows they are there and leaves them alone. That is not a failure — it is the arrangement.
Start here
Raise a mouse in a sterile bubble with no bacteria at all.
Sterile food, sterile air, sterile water, no bacteria anywhere in or on it. The mouse survives — and it needs about 30% more food than a normal mouse, has a thin and poorly developed gut wall, an underdeveloped immune system, and it is far easier to kill with an infection.
Removing every bacterium made the mouse worse. Why?
Because the bacteria were doing jobs. Extracting energy from fibre your own enzymes cannot touch, making vitamins you cannot make, occupying space so that harmful species have nowhere to settle, and training the immune system by being there. The mouse lost all of these at once.
Switch them off · five jobs
Take one job away and see what breaks
0 of 5 switched off
Every one of these is something your gut bacteria do that your own cells cannot.
Job 1
Fermenting fibre
Your own enzymes cannot break cellulose. Gut bacteria can, and the fatty acids they release are absorbed through the large intestine wall and used by your cells.
Without it: A share of the energy in your food — up to about a tenth for some people — simply leaves the body undigested. The germ-free mouse needs 30% more food for the same growth, and this is most of the reason.
Job 2
Making vitamins
Bacteria in the large intestine make vitamin K and several B vitamins as by-products of their own metabolism, and you absorb them.
Without it: Vitamin K deficiency, and blood that clots poorly. Newborn babies have almost no gut bacteria yet, which is exactly why they are given a vitamin K injection at birth.
Job 3
Occupying the space
Harmful species need somewhere to settle and something to eat. A gut already full of established bacteria offers neither.
Without it: Empty space and free food. This is why a course of antibiotics can be followed by an infection that the antibiotic was not treating — the competition was cleared away.
Job 4
Training the immune system
A developing immune system learns what to attack and what to leave alone by encountering harmless bacteria early. The gut community is most of that encounter.
Without it: An immune system that has never been calibrated — slower against real threats, and more likely to react to things that are not threats at all.
Job 5
Maintaining the gut wall
The fatty acids bacteria release are the preferred fuel of the cells lining the large intestine, and they signal those cells to keep the wall thick and the mucus layer intact.
Without it: A thin, poorly developed wall — measurable in the germ-free mouse, and a route for bacteria to cross where they should not.
All five off
You have just built the germ-free mouse.
It lives. It is simply worse at everything — more food for the same growth, a weaker gut wall, missing vitamins, and no defence in place when a harmful species arrives. The bacteria were not tolerated. They were part of the system.
Both sides of the deal
What each side gets, and what happens when the balance shifts.
What you get · Chemistry you do not have
Energy from fibre, vitamin K and B vitamins, a defended gut, a trained immune system and a maintained wall. Five jobs, none of which your own twenty thousand genes can do.
What they get · A warm, fed, stable habitat
Constant 37 °C, a steady supply of food arriving from above, no competition from outside, and no immune attack. From the bacteria’s point of view your large intestine is excellent property.
When it shifts · The same species, the wrong place
A useful community in the large intestine becomes a serious problem if it crosses into the blood, or if antibiotics clear the space and one harmful species takes over. What makes a bacterium dangerous is usually location and abundance, not identity.
Key fact
Most bacteria in your gut are helpful and a few can be harmful. Which they are depends on the species and on where they are — the same bacterium can be useful in the large intestine and dangerous in the blood.
Think again
“Bacteria are germs. Having bacteria inside you means you are ill.”
Of the many thousands of bacterial species that live on and in humans, the number that reliably cause disease is a small minority. The rest are neutral or useful, and the useful ones are doing work you cannot do without: about a tenth of the energy some people get from their food comes from bacteria fermenting fibre, and a large part of your vitamin K is made by bacteria in your own large intestine. The word germ is not a biological category at all — it is a word for “microorganism I do not want here”, and the here matters more than the what. E. coli lives harmlessly in almost every human large intestine; the same species in your bladder is a urinary infection and in your blood it can kill you. Nothing about the bacterium changed.
“One cell means simple, so gut bacteria cannot be doing anything complicated.”
You met this in Unicellular organisms and it comes back with a bigger consequence. Your gut bacteria collectively carry several million genes; you carry about twenty thousand. They can build enzymes for reactions no human cell can perform, which is exactly why they can digest cellulose and make vitamins and you cannot. It is not that they are helping with your chemistry. They are running chemistry you do not have.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · What they do for you
Which of these can your gut bacteria do that your own cells cannot?
Rung 2 · The one that catches people
A patient takes a broad-spectrum antibiotic for a chest infection and two weeks later develops severe diarrhoea from C. difficile. Why did clearing bacteria make a bacterial infection more likely?
Rung 3 · Explain the germ-free mouse
Explain why a germ-free mouse needs about 30% more food than a normal mouse, and why it is also more vulnerable to infection. Use at least three of the five jobs.
Rung 4 · Take it somewhere new
Cows digest grass; humans cannot. Cows have no cellulase enzyme of their own either. Using this lesson, explain how a cow manages it, and say what this suggests about the size of a cow’s gut compared with ours.
Key note
The large intestine holds trillions of bacteria. They ferment fibre your own enzymes cannot digest, make vitamin K and some B vitamins, occupy space that harmful species would otherwise take, and help the immune system develop. A few species can cause disease, and antibiotics disturb the whole community rather than only the target.
Going further
One of the more startling treatments in modern medicine is a faecal microbiota transplant: bacteria from a healthy donor's gut, screened and prepared, introduced into a patient whose own gut community has been destroyed — usually by antibiotics, allowing Clostridioides difficile to take over the empty space and cause a severe, stubborn infection. Antibiotics alone often fail, because they clear the space again for the same organism to reoccupy. Restoring a full community of competitors works in around nine cases out of ten. It is a treatment that only makes sense once you stop thinking of bacteria as contamination and start thinking of a gut community as an ecosystem with vacancies.
Before this lesson
Connects to
At GCSE this becomes
- Communicable disease, antibiotic resistance, and the role of the microbiome in health.
Where to next
- Next: The gas exchange system
Breathing and gas exchange
- Previous: Absorption and the small intestine
Ask Mr Badmus AI
Want to talk through why the same bacterium can be helpful or harmful?
Figures for bacterial numbers and gene counts are order-of-magnitude estimates from current research and are revised regularly. Nothing here is advice about antibiotics or probiotics — those decisions belong with a doctor.
Lesson content © MrBadmusAI.